This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-155603 filed on May 24, 2001 and Japanese Patent Application No. 2001-293064 filed on Sep. 26, 2001.
1. Field of the Invention
The present invention relates to an electric motor having a cooling fan received in a yoke.
2. Description of Related Art
One electric motor having a cooling fan is disclosed, for example, in Japanese Unexamined Patent Publication No. 2001-61257. The motor includes a yoke housing (hereinafter, simply referred to as a yoke) and a worm housing. The cooling fan is secured around a rotatable shaft within the yoke. When the cooling fan is rotated, the air in the interior of the motor is discharged through air outlet openings provided at one end of the yoke near the cooling fan. At the same time, external air is drawn into the interior of the yoke through an air inlet opening provided in the worm housing to circulate the air in the yoke to cool the interior of the yoke.
Another type of motor similar to the above motor has been also proposed. In this motor, the air outlet openings of the above motor are used as air inlet openings, and air outlet openings are formed in the other end (near a commutator and brushes) of the yoke. By rotating the cooling fan, the external air is drawn into the yoke through the air inlet openings and is discharged through the air outlet openings. In this motor, the relatively strong air is blown to a core (armature core) and a winding portion, which act as heat sources, to cool the core and the winding portion.
In such a motor, the core and the winding portion can be effectively cooled by the strong external air blown by the cooling fan. However, the external air is also blown to the electrically conductive components, such as the commutator and the brushes. Thus, if the external air has a high salt concentration and a high humidity, the commutator and the brushes can be corroded and degraded, causing an electrical conduction failure in the commutator and the brushes.
Furthermore, if this motor is a high power motor that generates a relatively large amount of heat, a total open area of the air inlet openings and a total open area of the air outlet openings should be increased to maintain a sufficient amount of air flow throughout the interior of the motor. However, some motors, such as vehicle motors, are often placed in an watery environment, such as a rainy road, where the water is often applied to the motors. In such a case, the increase in the total open area of the air inlet openings of the yoke and also the total open area of the air outlet openings of the yoke may cause intrusion of the water (e.g., salty water) or snow into the interior of the motor through the air inlet openings and the air outlet openings. If this happens, the components received in the interior of the motor may be corroded and degraded, causing a reduction in output power of the motor.
To address this disadvantage, it is conceivable to provide a water blocking cover around the yoke of the motor. However, in many cases, the water blocking cover can disadvantageously reduce the cooling performance of the motor. That is, if the water blocking cover surrounds the yoke of the motor, a flow passage of the air is formed between the exterior of the yoke and the cover. This causes the air discharged from the yoke through the air outlet openings of the yoke to be redrawn into the air inlet openings of the yoke through the flow passage of the air. In such a case, the air heated in the interior of the yoke is circulated between the space, which is formed between the yoke and the cover, and the interior of the yoke. Thus, temperature of the circulated air is gradually raised, resulting in the reduced cooling performance of the motor.
Thus, it is an objective of the present invention to address the above disadvantages. To achieve the objective of the present invention, there is provided an electric motor including an armature, a cooling fan, a yoke and a water blocking cover. The armature includes a rotatable shaft. The cooling fan is secured to the rotatable shaft of the armature to rotate integrally with the rotatable shaft. The yoke receives the armature and the cooling fan and has at least one air inlet opening and at least one air outlet opening. The at least one air inlet opening is located at a first axial end portion of the yoke and penetrates through a wall of the yoke to introduce air into an interior of the yoke upon rotation of the cooling fan. The at least one air outlet opening is located at a second axial end portion of the yoke and penetrates through the wall of the yoke to discharge the air from the interior of the yoke upon the rotation of the cooling fan. The water blocking cover substantially blocks intrusion of water into the interior of the yoke by substantially covering the at least one air inlet opening and the at least one air outlet opening of the yoke. The water blocking cover includes an intake duct, an exhaust duct and a communication restraining portion. The air is drawn from outside of the water blocking cover into the at least one air inlet opening through the intake duct upon the rotation of the cooling fan. The air is discharged from the at least one air outlet opening to the outside of the water blocking cover through the exhaust duct upon the rotation of the cooling fan. The communication restraining portion is located in a space between the water blocking cover and the yoke and restrains free communication of the air between the intake duct and the exhaust duct through the space between the water blocking cover and the yoke.
In place of the above water blocking cover, a first water blocking cover and a second water blocking cover can be provided. The first water blocking cover substantially blocks intrusion of water into the interior of the yoke through the at least one air inlet opening by substantially covering the at least one air inlet opening and includes an intake duct, through which the air is drawn from outside of the first water blocking cover into the at least one air inlet opening upon the rotation of the cooling fan. The second water blocking cover substantially blocks intrusion of the water into the interior of the yoke through the at least one air outlet opening by substantially covering the at least one air outlet opening and includes an exhaust duct, through which the air is discharged from the at least one air outlet opening to outside of the second water blocking cover upon the rotation of the cooling fan.